The familial hypercholesterolemias are a group of hereditary disorders characterized by elevation of plasma low-density lipoproteins (LDL) and premature atherosclerosis. Based on clinical, laboratory and family studies, we have identified several variants of the disorder. Studies in cultured skin fibroblasts have revealed abnormalities in the metabolism of LDL, of cholesterol and fatty acids. We plan to study fibroblasts from several variants in an effort to evolve a classification based on a profile of biochemical abnormalities detected. Methods to be used include the incorporation of acetate into various lipid classes; the activities of certain enzymes of cholesterol and fatty acid synthesis; the binding, internalization and degradation of LDL and visualization of the binding of ferritin-LDL by electron microscopy.